In order to purify automotive exhaust gases, various catalysts for purifying exhaust gas, such as oxidizing catalysts and three-way catalysts, have been used. These catalysts for purifying exhaust gas are those that are completed by loading a noble metal, such as Pt, Rh or Pd, onto a support comprising a porous oxide, such as γ-alumina. The porous-oxide species, or the noble-metal species have been used by combining them variously depending on their purposes.
For example, although Rh is a necessary and indispensable noble metal for catalyst for purifying exhaust gas because the reducing activity is high, there has been such a drawback that the activity degrades when being loaded in proximity to Pt. Hence, the following have been adopted: a method of turning a catalytic layer into two layers and then Pt and Rh are loaded respectively onto the respective layers, or a method of loading Pt and Rh respectively onto different oxide powders and thereafter mixing them; and thereby it has been carried out to load Pt and Rh separately to each other.
Moreover, it has been know that ceria, or ceria-zirconia solid solutions, and the like, exhibit oxygen sorbing/desorbing capacity (OSC). By including such an oxide exhibiting OSC, oxygen is stored when exhaust-gas atmosphere becomes oxygen rich; and the stored oxygen is released when the exhaust-gas atmosphere becomes oxygen poor. Therefore, it is possible to relax atmosphere fluctuation, and thereby the activities of three-way catalyst improve.
And, the following have come to be understood: the granular growth is suppressed in Pt loaded on ceria; and catalyst in which Rh is loaded on zirconia shows hydrogen generating capacity in exhaust gas, and accordingly the NOx purifying activity is high; and so forth. Consequently, when designing a catalyst for purifying exhaust gas, the combination of porous-oxide support species and noble-metal species have been becoming an extremely important factor.
However, in the above-described two-layer-structured catalyst for purifying exhaust gas, there has been such a problem that exhaust gases are less likely to reach down to the lower layer when the gas diffusibility in the upper layer is low and thereby the demonstration of performance resulting from catalytic metal that is loaded on the lower layer becomes insufficient.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2003-326,170 and Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-330,025, there are set forth a method of forming an upper layer of a slurry in which activated carbon that possesses large particle diameters to some extent, or the like, is mixed, and then forming an upper layer with large porosity by burning it out upon its calcination. In a catalyst for purifying exhaust gas, catalyst which possesses an upper layer being thus formed, the purifying performance improves, because the gas diffusibility in the upper layer improves considerably and thereby the contacting ability to noble metal also improves.
By the way, for the support substrate that is used in three-way catalyst, and the like, a honeycomb substrate that possesses an innumerable number of cellular passages have been used widely. This honeycomb substrate is formed of heat-resistant ceramic, such as cordierite, and has cross-sectionally quadrangular cellular passages. Although a coating layer is formed by wash coating a slurry onto such a honeycomb substrate, there has been such a phenomenon that the nearer it is coated onto the cornered portions of the cellular passages the thicker it is coated because of the slurry's surface tension. Accordingly, the gas diffusibility is low at the thick parts; in the case where a noble-metal loading amount is much, the loading density of catalytic noble metal becomes high at thin portions; and thereby the problem of granular growth arises when being employed.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-263,416 or Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-237,602, there is a description on using a honeycomb substrate that possesses cross-sectionally hexagonal cellular passages. When using a honeycomb substrate that possesses cross-sectionally hexagonal cellular passages like this, it is possible to relax the phenomenon that the nearer it is to the cornered portions the thicker it becomes.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2003-326,170;
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-330,025;
Patent Literature No. 3: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-263,416; and
Patent Literature No. 4: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-237,602